1. Field of the Invention
The present invention relates to an improved signal transmission fuse such as shock tube, of the type used for transmitting a detonation signal, and more particularly to an improved construction of such fuse.
2. Description of Related Art
Signal transmission fuses of the type commonly referred to as shock tube are well-known in the art. U.S. Pat. No. 3,590,739 issued Jul. 6, 1971 to Per-Anders Persson discloses a hollow elongated plastic tube having a pulverulent reactive substance, which may be constituted by a highly brisant explosive such as PETN, RDX, TNT or HMX, adhered in one manner or another to the interior wall of the shock tube.
U.S. Pat. No. 4,328,753 issued May 11, 1982 to L. Kristensen et al discloses a shock tube, described as a low energy fuse, in the form of a plastic tube comprised of concentric tubular plies of material. The inner or sub-tube is made of a polymeric material, such as an ionomeric plastic of the type sold under the trademark SURLYN by E. I. Du Pont Company, to which a pulverulent reactive material will cling. The sub-tube is surmounted by an outer tube made of a mechanically tougher material such as a polyamide, polypropylene, polybutene or other such polymer having satisfactory mechanical properties to withstand the stresses of deploying the fuse on a work site. The reactive material is a powdered mixture of an explosive such as cyclotetramethylene tetranitramine (HMX) and aluminum powder. The Patent discloses (column 2, line 1 et seq. and line 28 et seq.) that for a plastic tube having an outer diameter of 3 millimeters and an inner diameter of 1.3 millimeters, there should be a core loading of at least 2.7 grams of reactive material per square meter of the inner surface of the tube in order to insure that the requisite shock wave is transmitted through the tube upon initiation. It is disclosed as an advantage that the adhesive sub-tube permits the coating of reactive material to attain a core loading of up to about 7 grams per square meter of the inner surface of the tube (column 2, lines 64-66).
U.S. Pat. No. 4,607,573 issued Aug. 26, 1986 to G. R. Thureson et al discloses a laminated fuse comprising two or more laminated layers of material and a method of making the same including elongating the sub-tube after application of the pulverulent reactive material to the interior thereof to reduce both the wall thickness of the sub-tube and the loading thereon of reactive material per unit length ("core load"). An outer coating is applied to the outer surface of the elongated sub-tube to extend coextensively therewith and thereby provide a laminated tube having the layers thereof bonded securely to each other. Generally, the Thureson et al Patent discloses (column 3, line 9 et seq.) that the inner tube will have an average inside diameter of 0.017 to 0.070 inch (0.432 mm to 1.778 mm) and an outside diameter of 0.034 to 0.180 inch (0.864 mm to 4.57 mm) and an outer coating or layer applied over the inner or sub-tube. The Examples starting at column 5 of the patent show finished tubes (the inner or sub-tube with the overlying sheath or sheaths) having an outside diameter ("OD") of 0.150 inch (3.810 mm) and an inside diameter ("ID") of 0.051 inch (1.295 mm) in Example 1. Examples 2 and 3 each show a tube having a 0.118 inch (2.997 mm) OD and, respectively, 0.040 inch (1.016 mm) and 0.041 inch (1.041 mm) ID.
U.S. Pat. No. 5,212,341 issued May 18, 1993 to A. M. Osborne et al discloses multiple-layer, co-extruded shock tube having an inner layer or ply (sub-tube) having a thickness of less than 0.3 millimeter. It is stated that by making the sub-tube so thin a savings is effectuated by reducing the quantity of the more expensive (as compared to the material of the outer tube) material of which the powder-adherent inner tube is made. The Osborne et al patent, as does the above-mentioned U.S. Pat. No. 4,328,753, discloses at column 2, line 60 et seq., that at least 2.7 grams of reactive material per square meter of the tube inner surface is desired and the Examples at columns 3-4 disclose a tube having an outside diameter of 3.0 mm and an inside diameter of 1.1 mm (Example 1) and a tube having an outside diameter of 3.0 mm and an inside diameter of 1.2 mm (Example 2).